Knock-up plate lifting device and image forming apparatus having the same

ABSTRACT

A knock-up plate lifting device for lowering a knock-up plate while inserting and withdrawing a paper feeding cassette, and lifting the knock-up plate when the paper feeding cassette is completely mounted. The knock-up plate lifting device includes a mounting board. A first link is pivotably mounted by a first hinge to the mounting board and has arms extended away from the first hinge. A resilient member is connected to one of the arms of the first link. A second link is pivotably mounted by a second hinge to the mounting board and extends away from the second hinge by a predetermined angle. One arm of the second link is connected with the resilient member and the other arm moves up and down in accordance with the operation of the first hinge.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(a) of KoreanPatent Application No. 2004-81005, filed Oct. 11, 2004, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a paper feeding cassette for an imageforming apparatus. More particularly, the present invention relates to aknock-up plate lifting device for raising and lowering a knock-up plateof a paper feeding cassette.

2. Description of the Related Art

Generally, an image forming apparatus includes a paper feeding cassettefor receiving a stack of a printing medium to automatically supply theprinting medium, and a pickup roller for separating and conveying thestacked printing medium sheet by sheet. The paper feeding cassetteincludes a knock-up plate for biasing the stacked printing medium towardthe pickup roller. One end of the knock-up plate is hinged to the paperfeeding cassette while the other end is resiliently supported by aresilient member such as a spring to bias the stacked printing mediumtoward the pickup roller. However, since the knock-up plate isresiliently supported, it may collide with the pickup roller whileinserting and withdrawing the paper feeding cassette with respect to amain body of the image forming apparatus and thereby disturb theinsertion and withdrawal of the paper feeding cassette. Therefore, aknock-up plate lifting device is preferably configured to lower theknock-up plate during the insertion and withdrawal of the paper feedingcassette and lift the knock-up plate toward the pickup roller aftercompleting the insertion.

Referring to FIGS. 1 and 2, a conventional knock-up plate lifting device10 comprises a link 40, a resilient member 30, and a rectilinear-movingmember 20. The link 40 has a bent form and is fixed by a hinge 43 withina main body 70 (FIG. 3) of the image forming apparatus that the knock-upplate lifting device 10 is mounted in. The hinge 43 is formed at a jointportion of the link 40. The link 40 comprises a short arm 41 and a longarm 42 with respect to the hinge 43. An end of the short arm 41 isconnected to one end of the resilient member 30 whereas an end of thelong arm 42 has a guiding projection 51 for guiding vertical motion ofthe link 40. A guiding groove 52 is formed in front of the guidingprojection 51 to correspond to a moving path of the guiding projection51.

The resilient member 30 connects the link 40 and the rectilinear-movingmember 20. The long arm 42 of the link 40 moves up and down by apredetermined angle in accordance with the motion of therectilinear-moving member 20. A tension coil spring may be used for theresilient member 30.

The rectilinear-moving member 20 comprises a moving part 21 and aguiding part 22 for guiding the linear motion of the moving part 21. Theguiding part 22 is disposed within the main body 70 and the moving part21 is connected to the other end of the resilient member 30. On bothsidewalls 81 of the paper feeding cassette 80, operation projections 85are formed to push the moving part 21 of the rectilinear-moving member20 in a direction for mounting the paper feeding cassette 80, as shownin FIG. 3.

As the paper feeding cassette 80 is inserted into the main body 70 ofthe image forming apparatus, the operation projections 85 formed on thesidewalls 81 of the paper feeding cassette 80 push the moving part 21 ofthe rectilinear-moving part 20 in the mounting direction of the paperfeeding cassette 80, that is, the direction illustrated by the arrow Ain FIG. 2. Accordingly, tension is applied to the resilient member 30,thereby applying a force onto the short arm 41 of the link 40. The link40 is pivoted with respect to the hinge 43 by a predetermined angle bythe force, such that the end of the long arm 42 is lifted, asillustrated by the solid lines in FIG. 2. Therefore, if the end of thelong arm is connected to the knock-up plate 83 of the paper feedingcassette 80, the knock-up plate 83 of the paper feeding cassette 80 isalso lifted by the link 40. The motion of the long arm 42 is guided bythe guiding projection 51 and the guiding groove 52.

When the paper feeding cassette 80 is withdrawn from the main body 70,the operation projections 85 are separated from the moving part 21, and,accordingly, the force applied to the moving part 21 of therectilinear-moving member 20 is removed. The recovery force of theresilient member and the weight of the link 40 apply a force in theopposite direction, and, as a result, the moving part 21 is restored.After recovery, the long arm 42 of the link 40 is parallel with thebottom of the paper feeding cassette 80 (as shown in dashed lines), and,therefore, the knock-up plate 83 supported by the link 40 also becomesparallel with the bottom.

Therefore, when the paper feeding cassette 80 is withdrawn with respectto the main body 70, the knock-up plate 83 is lowered by the knock-upplate lifting device 10, and when the paper feeding cassette 80 iscompletely inserted into the main body 70, the knock-up plate 83 islifted so as to bring the printing medium stacked on the knock-up plate83 into tight contact with the pickup roller (not shown), therebyenabling the pickup of the printing medium.

However, in the conventional knock-up plate lifting device 10 describedabove, the lifting and lowering of the knock-up plate 83 depends on therectilinear motion of the rectilinear-moving part 20. Thus, if theguiding part 22 is jammed with foreign substances or if the slidingsurfaces of the guiding part 22 and the moving part 21 become rough dueto long-term use, the knock-up plate lifting device 10 may not operatesmoothly. This may cause difficulty in inserting and withdrawing thepaper feeding cassette 80 with respect to the main body 70, therebydeteriorating reliability of the product.

In addition, since the knock-up plate lifting device 10 is disposed at alower corner portion of the main body 70, assembly is complex.Therefore, productivity deteriorates and manufacturing costs increase.

Furthermore, the knock-up plate lifting device 10 is not easily seenfrom the outside of the main body 70. Therefore, maintenance and repairof the knock-up plate lifting device 10 is not easy, and this mayincrease maintenance costs.

Accordingly, there is a need for an improved knock-up plate liftingdevice that is more reliable and easier to manufacture and maintain.

SUMMARY OF THE INVENTION

An aspect of the present invention is to solve at least the aboveproblems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the present invention is toprovide an improved knock-up plate lifting device which is notinfluenced much by foreign substances located in the lifting device orby roughness of sliding surfaces in the lifting device.

Another aspect of the present invention is to provide a knock-up platelifting device which can be mounted on the sidewalls of the paperfeeding cassette to save assembly time and reduce manufacturing andmaintenance costs.

Yet another aspect of the present invention is to provide an imageforming apparatus with a knock-up plate that is not influenced much byforeign substances or by roughness of sliding surfaces and that reducesmanufacturing and maintenance costs.

In order to achieve the above-described aspects of the presentinvention, a knock-up plate lifting device is provided. The knock-upplate lifting device includes a mounting board. A first link ispivotably mounted by a first hinge to the mounting board. The first linkhas first and second arms extending in opposite directions away from thefirst hinge. A resilient member is connected to the first arm of thefirst link. A second link is pivotably mounted by a second hinge to themounting board. The second link has first and second arms extending awayfrom the second hinge. The first arm of the second link is connectedwith the resilient member so that the second arm moves up and down whenthe first link is pivoted.

The second arm of the second link is disposed at a lower position whenthe first link is vertical to the ground.

The arms of the second link have different lengths.

The knock-up plate lifting device further comprises a guiding projectionformed at the second arm of the second link, and a guiding groove formedat the mounting board to guide the vertical motion of the guidingprojection.

The knock-up plate lifting device further comprises an extension memberformed at an end of the first arm of the first link which is connectedto an end of the resilient member.

According to another aspect of the present invention, an image formingapparatus comprises a pickup roller mounted to a main body and a paperfeeding cassette removably mounted to the main body under the pickuproller. The cassette includes a knock-up plate for pushing a printingmedium toward the pickup roller. A first link is mounted to a sidewallof the paper feeding cassette by a first hinge and has first and secondarms extending in opposite directions from the first hinge. A resilientmember is connected to the first arm of the first link. A second link ismounted to the sidewall of the paper feeding cassette by a second hingeand has first and second arms. The first arm of the second link isconnected with the resilient member and the second arm of the secondlink lifts and lowers the knock-up plate in accordance with theoperation of the first link. An operation projection is formed on themain body to rotate the first link when the paper feeding cassette ismounted into the main body.

First links, resilient members, and second links are formed on bothsidewalls of the paper feeding cassette, and operation projections areformed on both sides of the main body to correspond to the first links.

The second arm of the second link is disposed at a lower position whenthe first link is vertical to the bottom of the paper feeding cassette.

The arms of the second link are different lengths.

The image forming apparatus further comprises a guiding projectionformed at the second arm of the second link to lift and lower theknock-up plate, and a guiding groove formed at the sidewall of the paperfeeding cassette to guide the vertical motion of the guiding projection.

The image forming apparatus further comprises an extension member formedat an end of the first arm of the first link which is connected to anend of the resilient member.

According to the embodiments of the present invention described above,the knock-up plate is operated by rotation of the first link, withoutusing a sliding device. Thus, the operation of the knock-up plate is notinfluenced much by foreign substances or by roughness of slidingsurfaces.

Also, since the knock-up plate lifting device can be mounted on thesidewalls of the paper feeding cassette, assembly time for the knock-upplate lifting device can be reduced as compared with the assembly timewhen the knock-up plate is mounted within the main body of the imageforming apparatus. Therefore, maintenance and manufacturing costs can besaved.

Additionally, the image forming apparatus applying the above knock-upplate lifting device has reduced maintenance and manufacturing costs.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The above and other objects, features, and advantages of certainembodiments of the present invention will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view of a conventional knock-up plate liftingdevice;

FIG. 2 is a schematic view illustrating the operation of the knock-upplate lifting device of FIG. 1;

FIG. 3 is a perspective view of an image forming apparatus including theknock-up plate lifting device of FIG. 1;

FIG. 4 is a schematic view of a knock-up plate according to anembodiment of the present invention;

FIG. 5 is a schematic view illustrating the operation of the knock-upplate of FIG. 4;

FIG. 6 is a perspective view showing an image forming apparatusincluding the knock-up plate lifting device according to an embodimentof the present invention, before the paper feeding cassette has beeninserted; and

FIG. 7 is a partial, sectional view illustrating the knock-up platelifting device when the paper feeding cassette is mounted in the imageforming apparatus of FIG. 6.

Throughout the drawings, the same drawing reference numerals will beunderstood to refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The matters defined in the description such as a detailed constructionand elements are provided to assist in a comprehensive understanding ofthe embodiments of the invention. Accordingly, those of ordinary skillin the art will recognize that various changes and modifications of theembodiments described herein can be made without departing from thescope and spirit of the invention. Also, descriptions of well-knownfunctions and constructions are omitted for clarity and conciseness.

Referring to FIGS. 4 and 5, a knock-up plate lifting device 110according to an embodiment of the present invention comprises a mountingboard 100, a first link 120, a second link 140, and a resilient member130.

The first and the second links 120 and 140 are mounted on the mountingboard 110 by first and second hinges 123 and 143, respectively, so thatthe links may rotate. By using a mounting board 110, the knock-up platelifting device 100 can be attached as a separate unit. The mountingboard 110 is attached to a place employing the knock-up plate liftingdevice 100, such as main body of a paper feeding cassette or an imageforming apparatus, by a fastening means (not shown) such as a screw.When a knock-up plate lifting device 100′ (FIG. 6) does not utilize adedicated mounting board 110, the first and the second links 120 and 140are directly hinged to a sidewall 181 (FIG. 6) of a paper feedingcassette 180 (FIG. 6) by the hinges 123 and 143.

The first link 120 is mounted by the first hinge 123 so that it canrotate with respect to the mounting board 110. An upper (or first) arm121 and a lower (or second) arm 122, extend in opposite directions awayfrom the first hinge 123. The first link 120 is vertical to the groundwhen no force is applied, and rotated when applied with force due tointerference between the upper arm 121 and an operation projection 171(FIG. 7). The lower arm 122 of the first link 120 is connected to an endof the resilient member 130. The lower arm 122 of the first link 120 mayfurther comprise an extension member 124 for connection with the end ofthe resilient member 130. The connection part 125 of the resilientmember 130, for connection with the extension member 124, is preferablydisposed so that the distance between the first hinge 123 and theconnection part 125 is almost constant whether the first link 120 is ina vertical posture and in a horizontal posture.

At a predetermined distance from the first link 120, the second link 140is rotatably mounted on the mounting board 110 by the second hinge 143.The second link 140 includes a right (or first) arm 141 and a left (orsecond) arm 142 extending away from the second hinge 143. The right arm141 (refer to FIG. 4) is connected to the other end of the resilientmember 130. The left arm 142 (refer to FIG. 4) is lifted and lowered bya predetermined angle according to the motion of the right arm 141. Asshown in FIG. 4, when the first link 120 is vertical to the ground, theleft arm 142 of the second link 140 is at a lower position, that is,horizontal with respect to the ground, and the left arm 142 is longerthan the right arm 141. The second link 140 is formed as a bent memberwherein the left arm 142 and the right arm 142 are bent at apredetermined angle with respect to each other, preferably in the rangeof approximately 90˜180°. A reinforcing piece 145 may be providedbetween the left and right arms 142 and 141 of the second link 140 tostrengthen the arms 141 and 142. As long as the left arm 142 of thesecond link 140 is lifted and lowered by the predetermined angle, thesecond link 140 may have various configurations according to the lengthof the first link 120, the positional relationship with the first link120, and the lifted height of the knock-up plate 183 (FIG. 6).

The left arm 142 of the second link 140 has a guiding projection 151while the mounting board 110 has a guiding groove 152 for guiding thevertical motion of the guiding projection 151. The guiding groove 152 isshaped to correspond to the arc of the movement of the end of the leftarm 142 of the second link 140. The lower end of the guiding groove 152is located where the left arm 142 of the second link 140 becomeshorizontal, and the upper end of the guiding groove 152 is located wherethe knock-up plate 183 is pressed to its maximum lift height. Theguiding groove 152 and the guiding projection 151 define the angle oflifting and lowering the second link 140 and stably guide the verticalmotion of the second link 140.

The resilient member 130 is interposed between the first and the secondlinks 120 and 140 to allow the left arm 142 of the second link 140 tomove up and down in accordance with rotation of the first link 120. Oneend of the resilient member 130 is connected to the lower arm 122 of thefirst link 120 or the extension member 124, while the other end of theresilient member 130 is connected to the right arm 141 of the secondlink 140. When the first link 120 is vertical, the resilient member 130applies no force to the second link 140. On the other hand, when thesecond link 140 is horizontal, the resilient member 130 lifts up theleft arm 142 of the second link 140 and applies a force for pushing aprinting medium stacked on the knock-up plate 183 into tight contactwith the pickup roller 175 (FIG. 7). Any resilient material capable oftransmitting a force, preferably a coil spring, can be used for theresilient member 130.

The operation of the knock-up plate lifting device 100 will now bedescribed with reference to FIG. 5. When a member for rotating the firstlink 120, such as the operation projection 171 (FIG. 6), interferes withthe upper arm 121 of the first link 120, the first link 120 rotates byapproximately 90° to be horizontal.

When the first link 120 rotates by approximately 90°, the resilientmember 130 in connection with the lower arm 122 of the first link 120pulls the right arm 141 of the second link 140 to the right with respectto FIG. 5. Therefore, the second link 140 rotates clockwise about thesecond hinge 143, thereby moving the left arm 142 of the second link 140upward with the assistance of the guiding projection 151 and the guidinggroove 152. If the guide projection 151 formed at the end of the leftarm 142 of the second link 140 is connected with the knock-up plate 183,the knock-up plate 183 is lifted up.

As the interference is removed from the first link 120, the guidingprojection 151 returns to the lower end of the guiding groove 152 due tothe weight of the second link 140. Accordingly, the left arm 142 of thesecond link 140 becomes horizontal and the first link 120 returns tovertical by its own weight and the recovery force of the resilientmember 130.

Thus, by the knock-up plate lifting device 100 according to anembodiment of the present invention, the left arm 142 of the second link140 can be moved up and down in accordance with the rotation of thefirst link 120.

FIG. 6 is a perspective view showing the image forming apparatuscomprising a knock-up plate lifting device 100′ according to anotherembodiment of the present invention. FIG. 7 is a partial, sectional viewfor illustrating the operation of the knock-up plate lifting device 100′shown in FIG. 6 when the paper feeding cassette is mounted in the imageforming apparatus.

The paper feeding cassette 180 comprises a knock-up plate lifting device100′ formed on a sidewall 181 of the cassette 180, and the knock-upplate 183.

In the previously discussed embodiment, a sub-assembly comprising themounting board 110 (FIG. 4), the first and the second links 120 and 140,and the hinges 123 and 143 is formed and the sub-assembly is fixed ontothe sidewall 181 of the paper feeding cassette 180 to thereby mount theknock-up plate lifting device 100. In this embodiment, the knock-upplate lifting device 100′ includes first and second links 120 and 140that are mounted directly on the sidewall 181 of the paper feedingcassette by hinges 123 and 143. The resilient member 130 connects thefirst and the second links 120 and 140. The first link 120 is mounteddirectly to the paper feeding cassette 180, but the structure of thefirst link 120 is the same as in the previous embodiment. Therefore, adetailed description of the first link 120 is not repeated. The guidinggroove 152 is formed on the sidewall 181 of the paper feeding cassette180 to guide the guiding projection 151 of the left arm 142 of thesecond link 140, and the guiding projection 151 of the second link 140is mounted to support the bottom of the knock-up plate 183. Since thesecond link 140 has the same structure as in the previous embodiment, adetailed description is not repeated. The knock-up plate lifting device100′ may be formed on only one sidewall 181 of the paper feedingcassette 180. Preferably, however, knock-up plate lifting devices 100′are provided on both sidewalls 181 to more stably support the verticalmotion of the knock-up plate 183.

The main body 170 of the image forming apparatus that mounts the paperfeeding cassette 180 has an operation projection 171 to operate theknock-up plate lifting device 100′. As the paper feeding cassette 180 isinserted in the main body 170, the operation projection 171 rotates thefirst link 120. When the paper feeding cassette 180 is completelyinserted, the operation projection 171 maintains the horizontal state ofthe first link 120, as shown in FIG. 7. When knock-up plate liftingdevices 100′ are formed on both sidewalls 181 of the paper feedingcassette 180, a second operation projection 171 is formed tocorresponding to the second knock-up plate lifting device.

With reference to FIGS. 6 and 7, the vertical motion of the knock-upplate 183 of the paper feeding cassette 180 in the above-structuredimage forming apparatus will now be described.

As shown in FIG. 6, when the paper feeding cassette 180 is drawn fromthe main body 170, the knock-up plate 183 is placed on the bottom of thepaper feeding cassette 180. In this state, the second link 140 of theknock-up plate lifting device 100′ is horizontal and the first link 120is vertical to the bottom of the paper feeding cassette 180.

When the paper feeding cassette 180 is inserted into the main body 170and advanced by a predetermined distance, the upper arm 121 of the firstlink 120 of the knock-up plate lifting device 100′ contacts theoperation projection 171 formed on the main body 170. As the paperfeeding cassette 180 is completely inserted, the first link 120 of theknock-up plate lifting device 100′ is kept horizontal by the operationprojection 171.

When the first link 120 rotates, tension is applied to the resilientmember 130. The resilient member is connected to the lower arm 122 ofthe first link 120 and the right arm 141 of the second link 140. Thus,the right arm 141 is pulled to the right with respect to FIG. 7 by theresilient member 130. Accordingly, the second link 140 rotates clockwiseabout the second hinge 143 and the guiding projection 151 formed at theend of the left arm 142 of the second link 140 moves upward along theguiding groove 152 of the paper feeding cassette 180. Consequently, theknock-up plate 183, which is supported by the guiding projection 151, islifted. Since the knock-up plate 183 is supported by the guidingprojection 151, the pickup roller 175 disposed above the knock-up plate183 is pushed with a certain force by the knock-up plate 183, as shownin FIG. 7.

When the paper feeding cassette 180 is withdrawn from the main body 170,the upper arm 121 of the first link 120 escapes from the interferencewith the operation projection 171, thereby rotating the first link 120clockwise. As the paper feeding cassette 180 is further withdrawn, thefirst link 120 completely escapes from the interference of the operationprojection 171 and becomes vertical to the bottom of the paper feedingcassette 180. Also, the left arm 142 of the second link 140 is moved tothe lower end of the guiding groove 152 by the weight of the second link140 and the knock-up plate 183 so that it is horizontal. The knock-upplate 183 is located at the bottom of the paper feeding cassette 180 atthis time and does not interfere with the pickup roller 175 mounted inthe main body 170 above the paper feeding cassette 180. Therefore, thepaper feeding cassette 180 can be withdrawn smoothly from the main body170 of the image forming apparatus.

After the paper feeding cassette 180 has been refilled with the printingmedium P and is remounted to the main body 170, the knock-up plate 183is lifted by the knock-up plate lifting device 100′ to bias the printingmedium P toward the pickup roller 175, thereby enabling the pickuproller 175 to pick up and convey the printing medium P sheet by sheet.

While the invention has been shown and described with reference tocertain embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

1. A knock-up plate lifting device comprising: a mounting board; a firstlink pivotably mounted by a first hinge to the mounting board and havingfirst and second arms extending away from the first hinge; a resilientmember connected to the first arm of the first link; and a second linkpivotably mounted by a second hinge to the mounting board and havingfirst and second arms extending away from the second hinge, wherein thefirst arm of the second link is connected with the resilient member andthe second arm moves up and down in accordance with the operation of thefirst hinge.
 2. The knock-up plate lifting device of claim 1, whereinthe second arm of the second link is disposed at a lower position whenthe first link is vertical to the ground.
 3. The knock-up plate liftingdevice of claim 2, wherein the first and second arms of the second linkhave different lengths.
 4. The knock-up plate lifting device of claim 3,wherein the knock-up plate lifting device further comprises: a guidingprojection formed at the second arm of the second link; and a guidinggroove formed at the mounting board to guide the motion of the guidingprojection.
 5. The knock-up plate lifting device of claim 3, furthercomprising: an extension member formed at the end of the first arm ofthe first link and connected to an end of the resilient member.
 6. Theknock-up plate lifting device of claim 1, wherein the first and secondarms of the second link are at a predetermined angle with respect toeach other.
 7. An image forming apparatus comprising: a pickup rollermounted to a main body; a paper feeding cassette removably mounted tothe main body under the pickup roller and having a knock-up plate forpushing a printing medium toward the pickup roller; a first link mountedto a sidewall of the paper feeding cassette by a first hinge and havingfirst and second arms extended in opposite directions from the firsthinge; a resilient member connected to the first arm of the first link;a second link mounted to the sidewall of the paper feeding cassette by asecond hinge and having a first arm connected with the resilient memberand a second arm lifting and lowering the knock-up plate in accordancewith the operation of the first hinge; and an operation projectionformed on the main body to rotate the first link when the paper feedingcassette is mounted into the main body.
 8. The image forming apparatusof claim 7, wherein both sidewalls of the paper feeding cassette includea first link, a resilient member, and a second link, and both sides ofthe main body includes an operation projection corresponding with thefirst links.
 9. The image forming apparatus of claim 7, wherein thesecond arm of the second link is disposed at a lower position when thefirst link is vertical to the bottom of the paper feeding cassette. 10.The image forming apparatus of claim 9, wherein the first and secondarms of the second link have different lengths.
 11. The image formingapparatus of claim 10, further comprising: a guiding projection formedat the second arm of the second link to lift and lower the knock-upplate; and a guiding groove formed at the sidewall of the paper feedingcassette to guide motion of the guiding projection.
 12. The imageforming apparatus of claim 7, further comprising: an extension memberformed on the end of the first arm of the first link and connected to anend of the resilient member.
 13. The image forming apparatus of claim 7,wherein the first and second arms of the second link are at apredetermined angle with respect to each other.
 14. An image formingapparatus comprising: a pickup roller mounted to a main body; a paperfeeding cassette removably mounted to the main body, a knock-up platemounted to the paper feeding cassette for pushing a printing mediumtoward the pickup roller; and a first knock-up plate lifting device,comprising: a first link pivotably mounted to a sidewall of the paperfeeding cassette, the first link having first and second arms extendingaway from the first hinge; a resilient member connected to the first armof the first link; a second link pivotably mounted to the sidewall ofthe paper feeding cassette, the second link having a first arm connectedwith the resilient member and a second arm for supporting the knock-upplate; and an operation projection formed on the main body to rotate thefirst link when the paper feeding cassette is mounted into the mainbody, wherein when the first link is rotated by the operationprojection, the second arm raises the knock-up plate.
 15. An imageforming apparatus according to claim 14, further comprising: a secondknock-up plate lifting device, comprising: a third link pivotablymounted to a second sidewall of the paper feeding cassette, the thirdlink having first and second arms extending away from the first hinge; asecond resilient member connected to the first arm of the third link; afourth link pivotably mounted to the second sidewall of the paperfeeding cassette, the fourth link having a first arm connected with thesecond resilient member and a second arm for supporting the knock-upplate; and a second operation projection formed on the main body torotate the third link when the paper feeding cassette is inserted intothe main body.
 16. An image forming apparatus according to claim 14,further comprising: a guiding projection formed at the second arm of thesecond link to support the knock-up plate; and a guiding groove formedon the sidewall of the paper feeding cassette to guide the motion of theguiding projection.
 16. An image forming apparatus according to claim14, wherein the first and second arms of the second link are at apredetermined angle with respect to each other.